1. Field of the Invention
The present invention relates to an inexpensive waveguide film type optical module which is formed by combining an optical waveguide film and a light emitting element such as a VCSEL, an optical waveguide film which is used in the waveguide film type optical module and a manufacturing method thereof.
2. Description Related Art
As a method for manufacturing a polymer optical waveguide, there have been proposed (1) a method in which monomer is impregnated into a film, a core portion is selectively exposed to change a refractive index and the film is laminated to the core portion (a selective polymerization method), (2) a method in which a core layer and a clad layer are coated with a material and a clad portion is formed using reactive ion etching (a RIE method), (3) a method which uses a photolithography method by performing exposure and developing using ultraviolet curable resin which is formed by adding a photosensitive material into the inside of a polymer material (a direct exposure method), (4) a method which utilizes injection molding, (5) a method in which a core layer and a clad layer are applied with a material and, thereafter, a core portion is exposed so as to change a refractive index of the core portion (photo bleaching method) and the like.
However, the selective polymerization method (1) has a drawback with respect to the lamination of the film, the methods (2), (3) become costly due to the use of the photolithography method, and the method (4) has a drawback with respect to the accuracy of an obtained core diameter. Further, the method (5) has a drawback that it is difficult to obtain the sufficient difference in refractive index between the core layer and the clad layer.
Currently, although the methods which exhibit the excellent performance in terms of practical use are the methods (2), (3), these methods have the above-mentioned drawbacks on cost. Further, any one of the methods (1) to (5) is not applicable to the formation of the polymer optical waveguide on a flexible plastic substrate having a large area.
In view of the above circumstances, inventors of the present invention have invented and have filed patent applications on a manufacturing method of a polymer optical waveguide which uses a mold as a method completely different from the above-mentioned conventional manufacturing methods of polymer optical waveguide (see, Japanese Patent Laid-Open Publication Nos. 2004-29507, 2004-86144, and 2004-109927). This method is extremely simple, can be performed at a low cost, and can manufacture the polymer optical waveguides on a mass production basis. Further, in spite of the easiness of manufacturing, the method can manufacture the polymer optical waveguides which exhibit a small waveguide loss and also can easily manufacture the polymer optical waveguides having any pattern shape provided that the mold can be formed. Still further, although it has been difficult to form the optical waveguide on the flexible substrate conventionally, this method has succeeded in the formation of the optical waveguide on the flexible substrate.
Here, recently, in the IC technique and the LSI technique, to enhance the operation speed and the integration density, the use of optical wiring between equipments, between boards in the inside of an equipment or in the inside of a chip in place of the electric wiring with high density has been attracting attentions.
As an element for optical wiring, for example, an optical element having the following constitution is described in Japanese Patent Laid-Open Publication No. 2000-39530. That is, in this patent document, a polymer optical waveguide having a core and a clad which wraps the core is provided with a light emitting element and a light receiving element in the core/clad stacking direction, and further, includes an incident-side mirror which allows light from the light emitting element to be incident on a core and an exit-side mirror which allows light exited from the core to enter into a light receiving element. In such an optical element, at portions corresponding to an optical path from the light emitting element to the incident-side mirror and an optical path from the exit-side mirror to the light receiving element, the clad layer is formed in a recessed shape so as to converge the light from the light emitting element and the light from the exit-side mirror. Further, an optical element having the following constitution is described in Japanese Patent Laid-Open Publication No. 2000-39531. That is, in this patent document, in the optical element which allows light from a light emitting element to be incident on a core end face of a polymer optical waveguide having a core and a clad which wraps the core, a light incident end face of the core is formed to have a convex shape which projects toward the light emitting element so as to converge the light from the light emitting element whereby the waveguide loss is suppressed.
Further, Japanese Patent Laid-Open Publication No. 2000-235127 describes an optoelectronic integrated circuit in which a polymer optical waveguide circuit is directly assembled to an optoelectronic fusion printed circuit board which integrates electronic elements and optical elements.
In the above-mentioned optical wiring, it is considered that if the above-mentioned elements could be mounted and could be incorporated into the inside of the apparatus, the degree of freedom in designing the assembling of the optical wiring could be increased and, as a result, compact and small light receiving and emitting elements could be provided.
However, the methods which have been proposed heretofore form a 90° folding mirror and hence, it is necessary to embed a mirror portion, and it is necessary to perform the highly accurate alignment at the time of laminating the optical waveguide and the light receiving and emitting elements thus giving rise to a serious drawback that the cost for mounting is pushed up.
On the other hand, in the optical module which is served for guiding the light of the light emitting element such as the VCSEL or the like to a connector to be connected with an optical fiber, an optical module of a type which couples the light through a micro lens and a 45 degree mirror is generally used. Such a constitution, however, requires one or two micro lenses on the light-emitting-element side and also at least one micro lens on the optical-fiber side whereby the cost necessary for forming the micro lenses and for aligning the optical axes of the micro lenses are pushed up. Further, in reflecting the light which propagates in space on a 45° mirror surface, to enhance the reflection efficiency, it is necessary to form an aluminum film by vapor deposition. This also pushes up the cost.
Accordingly, the optical module which uses an optical waveguide film has been attracting attentions in view of the lowering of the cost. This is because that when the end face of the optical waveguide film is formed into a 45° mirror surface and is directly adhered to the planar light emitting element such as the VCSEL or the like, the coupling which requires no micro lenses can be realized. Here, as a method for forming the optical waveguide film, the manufacturing method which is disclosed in the above-mentioned patent documents (Japanese Patent Laid-Open Publication Nos. 2004-29507, 2004-86144 and 2004-109927) is advantageous for lowering the cost.
However, the light output of the laser element such as the VCSEL or the like fluctuates due to an ambient temperature or the like, for example. To obtain the stable light output, it is necessary to monitor and feedback the light output per se. In this respect, in case of the optical waveguide module, efforts have been made including the provision of a branch waveguide for taking out and monitoring a portion of the light output. When the multiple light emitting points such as a 1×4 VCSEL array are used, it is difficult to ensure spaces for arranging photo detectors for monitoring (PD) and to connect waveguides branched to the spaces with the photo detectors for monitoring. When the photo detectors for monitoring are arranged on a side surface of the optical waveguide film, for example, by branching the branch waveguides from the waveguide which is coupled to two external light emitting points, it is possible to easily guide a portion of the output light to the photo detectors for monitoring. However, to branch the branch waveguides from the waveguide which is connected with two internal light emitting points to take out the light to the outside, it is necessary to make the branch waveguides intersect the external waveguide. Although a crosstalk is hardly generated when the waveguides orthogonally intersect each other at the intersecting portion, a slight waveguide loss is generated at the portion with respect to the external waveguide thus giving rise to a drawback that output characteristics differ between the inside and the outside. In this case, when the photo detectors for monitoring are collectively arranged outside in a 1×4 mode for lowering the cost, there arises a drawback that the number of intersections of the monitor waveguides is increased thus arising the possibility that the loss is increased and the output characteristics of the array waveguides differ.